This invention relates to means for detecting a loss of vacuum in any one of the vacuum-type circuit interrupters of a polyphase a.c. vacuum circuit breaker and, more particularly, relates to detecting means of this type which is very simple and inexpensive and relies for its operation to a large extent upon components already present in typical switchgear comprising a circuit breaker of the type used for protecting medium voltage three-phase a.c. power distribution circuits. Such switchgear is usually referred to as medium voltage metal-clad switchgear. Medium voltage denotes a voltage rating of between 2.4 kV and 38 kV.
Typical switchgear of the above type includes overcurrent-sensing means comprising three current transformer secondaries, one for each phase, connected in parallel with each other across a shorting conductor. In each of the parallel-connected branches of this overcurrent-sensing circuit and in series with the associated current transformer secondary, there is an input circuit of an overcurrent relay. The current transformer secondary for each phase delivers to the associated overcurrent relay a current substantially proportional to the primary current through the associated phase conductor. This secondary current flows through the series combination of the current transformer secondary, the input circuit of the associated overcurrent relay, and the shorting conductor. When the phase current exceeds a predetermined value, the associated overcurrent relay responds to the resulting increased secondary current by operating to initiate tripping of the circuit breaker.